The engines for outboard motor units and the like are generally of the two-cycle type with pressurized crankcase charging of the cylinders. The charge is introduced in either of two distinct methods known respectively as cross charging and scavenging and as loop charging and scavenging. Conventional cross charging and scavenging permits simplified manufacturing and minimizing of cost. In such systems, a deflector piston is employed to properly expose the exhaust port and the input port, which are located on opposite sides of the cylinder. The input charge, which may be a fuel-air charge or only air in fuel injection systems, is derived from the pressurized crankcase and moves across the piston and is then deflected upward to scavenge the exhaust gases while introducing the new charge. Although simple and relatively inexpensive, the system does not provide a highly efficient and effective scavenging and charging flow.
Loop scavenging is generally more efficient and thus produces a greater power output per cubic inch of piston displacement with a smaller fuel usage per horsepower per hour consumption when compared to cross scavenged engines. In loop scavenging, a pair of side input ports oppositely located in the cylinder directs the charges toward the rear of the cylinder and, with a finger port, develops a loop path through the cylinder with a wave moving from the back of the cylinder up the combustion chamber then back down to the exhaust port on the opposite side of the cylinder. Thus, the incoming charges meet with each other and with the upward charge from the finger port adjacent the back wall of the cylinder, sweep upwardly across the back of the cylinder, and then over and downward in a distinct loop to the exhaust port. Although more efficient, the opposed dual input porting increases the complexity and the cost of casting the cylinder block.
In some loop charged engines, such as that disclosed in U.S. Pat. No. 4,092,958, the charging ports and their associated transfer passages are defined by "blister" type cylinder liners which are integrally cast into the block. Other loop charged engines have used sand cores to form the transfer passages.
U.S. Pat. Nos. 3,149,383 and 2,288,902 disclose die cast cylinder blocks wherein the forming dies are withdrawn along the cylinder axes. These arrangements do not permit optimum shaping of the transfer passages, since no contouring of the lower surfaces of the transfer passage can be achieved.
U.S. Pat. No. 2,227,500 discloses a cross scavenged, two-cycle engine having a two cylinder die cast cylinder block with one transfer passage for each cylinder. The transfer passage and exhaust ports are directly opposed on opposite sides of the cylinder block and a cover is provided to complete the transfer passage on each cylinder.
Another block arrangement, suitable to allow die casting of a cross scavenged engine is shown in FIG. 7 of U.S. Pat. No. 2,731,960. In this engine the forming die for the transfer passage and crankcase is withdrawn laterally in a direction parallel to the crankshaft. A cover for the transfer passage is formed integrally with a crankshaft bearing cage.
Finally, U.S. Pat. No. 2,190,011 discloses several two-cycle engines in which lateral inserts are used to form the upper portion of the transfer passage.